Problem: 7 erasers cost $7.49. Which equation would help determine the cost of 15 erasers?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 7 erasers. We want to know the cost of 15 erasers. We can write the numbers of erasers as a proportion: $\dfrac{7}{15}$ We know 7 erasers costs $7.49. We can let $x$ represent the unknown cost of 15 erasers. The proportion of these costs can be expressed as: $\dfrac{\$7.49}{x}$ The cost changes along with the number of erasers purchased, and so the two proportions are equivalent. $\dfrac{7}{15} = \dfrac{\$7.49}{x}$